In a typical conventional method of manufacturing can ends, a sheet metal blank is positioned between a pair of dies which are moved to shear an edge of the blank, after which a punch descends to draw the now circular blank about an annular ring into a can end shell having a peripheral flange, a frustoconical wall, and an end panel connected to the frustoconical wall by an annular curved or radiused wall portion defining a radius of curvature between the frustoconical wall and the end panel. The can end shell is then removed from the first set of dies and inserted into a second set of dies, in which the peripheral flange is curled into a downward peripheral flange suitable for double seaming operations. Subsequently, the end shell is then placed between another pair of dies, which, when moved towards each other, form the curved or radiused connecting wall and the end panel of the shell into a domed central panel with a surrounding annular reinforcing channel or groove connecting the central panel to the frustoconical wall.
More recently, the conventional pairs of dies have been replaced by sets of concentric tools, allowing the sequential operations of drawing the can end shell from a flat circular blank, and then re-forming it to provide the domed central panel and annular reinforcing channel or groove, to be carried out at a single forming station, in a single manufacturing operation.
In conjunction with the advanced tooling and manufacturing techniques available for manufacturing such can ends, the design of the can ends has also developed. In general, there has been a drive to improve the strength of the shape of the can end, so that the thickness of the material from which the can end is made can be reduced whilst maintaining an equivalent pressure performance. As well as improving the absolute strength of the can ends, it is also necessary to ensure that the can ends exhibit an appropriate failure mode in the event that the internal pressure within the can should exceed that for which the can end is rated, for example due to handling and processing conditions to which the can is subjected, or as a result of the can being dropped, etc.
Some of the changes to can end geometry, in order to try to obtain an approved can end performance, include varying the configuration of the chuckwall, varying the configuration of the annular reinforcing groove or channel, and varying the configuration of the centre panel and the panel wall connecting the centre panel to the annular groove or channel.
Features introduced into the can end structure to improve the can end pressure performance can include bends, kinks and double angles in the chuckwall, changes in the radius of curvature at the base of the countersink, and the provision of stepped or otherwise complex panel wall structures. In certain arrangements, the chuckwall may even extend radially inwardly over the base of the countersink or annular reinforcing channel or groove, so as to create a concavo-convex wall structure. In such cases, the concavo-convex structure may be formed as a series of folds or otherwise as adjacent and oppositely curved reinforcing beads.
However, the more complicated the can end structure, the greater the amount of processing which the material of the original blank has to undergo in order to be formed into the desired end shape. The more the material is processed, the greater the amount of thinning that the material is likely to suffer due to the processing, in particular in the step of drawing the circular blank so as to form a can end shell. Thinning of the material during drawing reduces the strength of the material locally where the thinning occurs. Consequently, the overall strength of the can end is reduced as a result.
It will also be appreciated that the material from which can ends are manufactured is never truly homogeneous, exhibiting imperfections and variations in the crystalline or macromolecular structure and the like. Another result of excessive thinning of the material during the drawing and re-forming of the blank to form a can end is to exaggerate imperfections in the original blank material, which may lead to localised failure of the can end. As a result, unless very high quality control systems are put in place, it is hard to maintain the production of such can ends within desired tolerances, such that an unacceptably high number of defective can ends may be produced. However, since the presence or otherwise of imperfections and inhomogeneities in the blank material from which the can ends are made is an entirely random phenomenon, they cannot readily be predicted, and so any can end design or manufacturing process which results in the production of defective can ends in this manner is not acceptable.
It would thus be desirable to provide a method of manufacturing can ends which does not exhibit excessive thinning in the material from which the can end is drawn and re-formed, and can at the same time reduce the quantity of blank material used while maintaining the strength, pressure and failure performance of existing can ends.